The present invention relates to a rear projection screen for use in a projection television set and the like.
A shown in FIG. 1, a rear projector used in a projection television set is adapted to expand light emerging from a light source (P), e.g. a CRT, by means of a lens system (L), project the light from the rear side of a screen (S), and allow a viewer to observe a picture from the opposite side of the screen (S). However, if the distance from the light source (P) to the screen (S) is prolonged as shown, the projector becomes large in size. In practice, therefore, a method is adopted whereby one to three mirrors (M) are combined, and the light is projected after being reflected on them once. However, there have been drawbacks in that, in the method of FIG. 2(A), the height of the projector becomes large, while, in the methods of FIGS. 2(B) and 2(C), it cannot be said positively that the projectors have been made sufficiently compact in terms of the height and depth.
Many types of screens used for such a projector are provided with a circular Fresnel lens on the incidence side, thereby providing a means for allowing every corner of the screen to be brightly when viewed. However, since lens surfaces (a) are continuously formed via non-lens surfaces (b), as shown in FIG. 3, this circular Fresnel lens has a drawback in that the light incident upon the non-lens surfaces (b) indicated by the oblique lines do not converge, thereby reducing the efficiency of the Fresnel lens and adversely affecting the resolution. To prevent this from occurring, a measure is adopted in some cases to dispose the lens surfaces of the Fresnel lens to receive the light from the flat surface and to combine the Fresnel lens with a lenticular lens. However, such an arrangement of the screen with two lenses has disadvantages since the structure of the screen becomes complicated, the picture fades due to the flare occurring between the two lenses, and the efficiency of utilization of the light declines.
In addition, as a screen apparatus which is capable of reducing the depth of a projector, those which are disclosed in Japanese Patent Unexamined Publication Nos. 57120/1983 and 9649/1984 are known, which propose to introduce the light obliquely with respect to a screen, thereby reducing the depth of a projector system. However, since these apparatuses make use of the refraction of a lens, there has been a limit to enlarging the angle of incidence.
To overcome these drawbacks, a rear projection screen has been proposed for observing an image by introducing light at a sharp angle from the rear, characterized in that a plurality of prisms are provided in parallel with each other on the light incidence surface, each prism having a total reflection surface, so that the incident light is totally reflected on the total reflection surface and is then emerged from the front viewing surface of the screen (Japanese Patent Application No. 29964/1984).
Furthermore, prior to the above-described proposal made by the applicants, a projector having a similar arrangement is known (Japanese Patent Unexamined Publication No. 109481/1982).
Thanks to the above-mentioned method including the arrangements proposed by the applicants, it has become possible to provide a rear projection screen enabling the projector to become compact by reducing the dimensions in depth and height and to be bright without reducing the resolving power. Such methods have shortcomings in that even if it is possible to control the light in one axial direction, e.g. in the vertical direction, it is not possible to simultaneously control the light in the other direction, i.e., the horizontal direction.
As a result of carrying out a study to simultaneously control the light in the vertical and horizontal directions, the present invention proposes a screen having a uniform brightness.
To this end, the present invention provides a screen for viewing an image by introducing light from the rear side thereof at a sharp angle, characterized in that the rear surface of this screen has a plurality of elongated prisms disposed along curved lines, including elliptic and parabolic curved lines, each prism having an internal reflection surface extending along curved line, the internal reflection surface reflecting the light received by the rear surface so that the light is transmitted through the light transmitting medium and is then emerges from the front viewing surface.
It is to be understood that the term "curved lines" referred to in the present invention is applicable to hyperbolic lines in addition to the aforementioned arcuate curved, elliptic, and parabolic lines.
Description of the preferred embodiments of the present invention will be made hereinafter with reference to the accompanying drawings.